Method of Manufacturing Ti-Containing Austenitic Stainless Steel Sheet by Twin Roll Strip Caster

ABSTRACT

There is provided a method of manufacturing a titanium (Ti)-containing austenitic stainless steel sheet having a high degree of surface quality by using a twin roll strip caster. The method includes controlling a composition of molten steel such that a TiN precipitation temperature may be higher than a temperature of the molten steel in a tundish (T/D) by at least 50° C. (ΔT≧50° C.), the TiN precipitation temperature being defined by the following formula 2: 
       Log(N%)=−19,755/(T+273)+7.78+0.07[% Ti]−log [% Ti]+
 
       0.045[% Cr] 
       T(° C.)=−19,755/log(N%)−7.78−0.07(%Ti)+log(%ti)−0.045(%Cr)−273  [Formula 2]

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No.10-2013-0160262 filed on Dec. 20, 2013, with the Korean IntellectualProperty Office, the disclosure of which is incorporated herein byreference.

BACKGROUND

The present disclosure relates to a method of manufacturing a titanium(Ti)-containing austenitic stainless steel sheet using a twin roll stripcaster, and more particularly, to a method of manufacturing aTi-containing austenitic stainless steel sheet having a high degree ofsurface quality by directly casting molten steel into a strip in a twinroll strip caster instead of using a conventional continuous castingprocess, so as to prevent a nozzle clogging and the formation of largesurface defects (caused by TiN and oxide inclusions).

Titanium (Ti)-containing austenitic stainless steel has a high Ticontent of about 0.2% to about 0.5%, and Ti nitrides or oxides are verylikely to be formed if Ti-containing austenitic stainless steel is castin a general continuous casting process. Mixtures of such Ti nitridesand oxides are known to cause clogging of immersion nozzles of ladles ortundishes.

In addition, large clusters formed of Ti nitrides cause many surfacedefects, and thus most Ti-containing steel sheets are subjected to apost-process such as a hot-rolling process after grinding upper/lowersurfaces thereof. If such defects remain on hot-rolled coils, finalproducts may only be produced after performing a coiling grindingprocess or a cold-rolling process on the hot-rolled coils. Due to thesereasons, additional process costs and burdens are increased, and processyields are decreased, to lower profits. Thus, there is a need for amethod of solving such problems.

In general, titanium (Ti) is included in highly corrosion-resistantTi-containing stainless steel in an amount of 0.2% to 0.5% so as to fixcarbon in the form of TiC and thus to prevent chromium (Cr), animportant element guaranteeing the corrosion resistance of stainlesssteel, from precipitating in the form of Cr₂₃C₆. Like general stainlesssteel, Ti-containing stainless steel may be produced through processesusing an electric furnace, a refining furnace, a ladle treatment unit,and a continuous caster. That is, in the related art, Ti-containingstainless steel is formed as a slab by a continuous casting method, andthe slab is hot-rolled to form a strip having a thickness of 2 mm to 6mm.

FIG. 1 is a schematic view illustrating a continuous casting process ofthe related art. As shown in FIG. 1, a continuous casting process may beperformed using a ladle 2 containing molten steel 1 produced through asteel making process, a tundish 4 disposed between the ladle 2 and amold 8 as a buffer, the mold 8 for producing slabs, and a secondarycooling table 9. Molten steel 1 is formed into a slab while passingthough the above-mentioned devices, and then the slab is reheated andhot-rolled into a strip in a region denoted by a box in FIG. 1. Then,the strip is water-cooled and coiled.

For example, scraps and a ferro alloy are melted into molten steel in anelectric furnace, and the molten steel is refined in a refining furnaceto remove carbon, phosphorous, and sulfur therefrom. Thereafter, thetemperature and minor components of the molten metal are adjusted in aladle treatment unit according to conditions for a casting process.After the temperature and minor component adjustment, the molten steelis continuously cast and formed as a hot-rolled coil product.

However, the addition of titanium (Ti) results in the formation of Tioxides (e.g. TiO₂) and Ti nitrides (e.g. TiN) because titanium (Ti) hasa high affinity for oxygen and nitrogen, and since Ti oxides and Tinitrides have very high melting points, Ti oxides and Ti nitrides mayclog nozzles during a casting process and degrade the surface quality ofproducts. Particularly, if nozzles are clogged during a casting process,molten steel may not be supplied to a tundish or a caster, and thus itmay be impossible to perform the casting process. That is, due to such asituation, the production of products may be interrupted.

Materials causing the clogging of nozzles may be included in moltensteel as large inclusions. In this case, the surface quality of productsmay be very poor. As described above, if nozzles are clogged during acasting process, product quality as well as productivity may be largelyaffected.

In the related art, the following methods have been proposed to preventthe clogging of nozzles.

First, a method of decreasing the amount of titanium (Ti) has beenproposed. That is, the amount of titanium (Ti) is decreased to reducethe formation of Ti oxides and Ti nitrides. In this case, however, theamount of carbon (C) also has to be reduced. The basic design concept ofTi-containing stainless steel is to increase the content of titanium(Ti) to a certain value or greater according to the content of carbon(C) so as to obtain a desired degree of corrosion resistance. That is,the ratio of Ti/C, termed a Ti stabilization ratio, is set to be about 5to about 10, generally at least 5. Therefore, if the addition oftitanium (Ti) is reduced, the amount of carbon also has to be reduced tosatisfy the Ti stabilization ratio. In this case, a large amount ofoxygen may have to be supplied to a refining furnace to decrease theamount of carbon. Due to this, a larger amount of oxygen may bedissolved in molten steel, and thus the formation of Ti oxides may beunexpectedly increased. In addition, since the period of time necessaryfor a refining process is increased, the temperature of molten steel mayincrease, and thus a considerable amount of coolant may be necessary forcooling the molten steel after the molten steel is discharged from therefining furnace. In this case, the molten steel may easily make contactwith ambient air while being cooled by the coolant and thus may bere-oxidized to cause the formation of large amounts of inclusions.

Secondly, in another method of reducing the amount of titanium (Ti), alarge amount of aluminum (Al), having a relatively higher affinity foroxygen than titanium (Ti), is added before titanium (Ti) is added so asto remove dissolved oxygen. This method effectively prevents theformation of Ti oxides. However, a large amount of Al₂O₃ that affectsthe surface quality of products more negatively than Ti oxides is formedas a result of oxygen removal using aluminum (Al), and thus a process ofadding calcium (Ca) to molten steel is required to convert Al₂O₃ intoCaO—Al₂O₃ having a less negative effect. However, it is difficult tocontrol the content of calcium (Ca) due to a high degree of volatilityof calcium (Ca). In addition, if the content of calcium (Ca) isexcessively low or high, inclusions having a desired shape may not beformed, and thus surface defects may increase. Particularly, if thecontent of calcium (Ca) is excessively high, calcium (Ca) included inmolten steel may react with Al₂O₃ included in a refractory material of astopper that is used to adjust the supply of molten steel from a tundishto a caster, and as a result, a compound having a low melting point,CaO—Al₂O₃, is formed to increase erosion of the stopper. In this case,the supply of molten steel may become unstable, and thus the quality ofproducts may be lowered.

SUMMARY OF THE INVENTION

An aspect of the present disclosure may provide a method ofmanufacturing a titanium (Ti)-containing austenitic stainless steelsheet by using a twin roll strip caster so as to suppress the formationof TiN and effectively prevent nozzle clogging occurring in a generalcontinuous casting process, thereby ensuring casting stability and ahigh degree of product surface quality.

However, aspects of the present disclosure are not limited thereto.Additional aspects will be set forth in part in the description whichfollows, and will be apparent from the description to those of ordinaryskill in the related art.

According to an aspect of the present disclosure, there may be provideda method of manufacturing a titanium (Ti)-containing austeniticstainless steel sheet having a high degree of surface quality by using atwin roll strip casting process, the method including controlling acomposition of molten steel such that a TiN precipitation temperaturemay be higher than a temperature of the molten steel in a tundish (T/D)by at least 50° C. (ΔT≧50° C.), the TiN precipitation temperature beingdefined by the following formula 2:

$\begin{matrix}{{{{Log}\left( {N\mspace{14mu} \%} \right)} = {{{- 19},{755/\left( {T + 273} \right)}} + 7.78 + {0.07\left\lbrack {\% \mspace{14mu} {Ti}} \right\rbrack} - {\log \left\lbrack {\% \mspace{14mu} {Ti}} \right\rbrack} + {0.045\left\lbrack {\% \mspace{14mu} {Cr}} \right\rbrack}}}{{T\left( {{^\circ}\mspace{14mu} {C.}} \right)} = {\frac{{- 19},755}{\begin{matrix}{{\log \left( {N\mspace{14mu} \%} \right)} - 7.78 - {0.07\left( {\% \mspace{14mu} {Ti}} \right)} +} \\{{\log \left( {\% \mspace{14mu} {Ti}} \right)} - {0.045\left( {\% \mspace{14mu} {Cr}} \right)}}\end{matrix}} - 273}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

The molten steel may include, by weight %, carbon (C): 0.025% to 0.055%,silicon (Si): 0.25% to 0.55%, manganese (Mn): 1.5% to 1.8%, chromium(Cr): 17.1% to 17.7%, nickel (Ni): 9.25% to 9.65%, titanium (Ti): 0.2%to 0.5%, nitrogen (N): 0.025% or less, and the balance of iron (Fe) andinevitable impurities.

The molten steel may have a Ti/C ratio of 8 or greater.

BRIEF DESCRIPTION OF DRAWINGS

The above and other aspects, features and other advantages of thepresent disclosure will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic view illustrating a continuous casting process ofthe related art;

FIG. 2 is a schematic view illustrating a strip caster according to anexemplary embodiment of the present disclosure;

FIG. 3 is an electron microscope image of a TiN+TiO₂ cluster of acomplex oxynitride;

FIG. 4 illustrates an image of a surface of a cast material and an imageof an inside of a tundish nozzle in an comparative example; and

FIG. 5 illustrates an image of a surface of a cast material and an imageof an inside of a tundish nozzle in an inventive example.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of the present disclosure will now be described indetail with reference to the accompanying drawings.

The disclosure may, however, be exemplified in many different forms andshould not be construed as being limited to the specific embodiments setforth herein. Rather, these embodiments are provided so that thisdisclosure will be thorough and complete, and will fully convey thescope of the disclosure to those skilled in the art.

In the drawings, the shapes and dimensions of elements may beexaggerated for clarity, and the same reference numerals will be usedthroughout to designate the same or like elements.

FIG. 2 is a schematic view illustrating a twin roll strip caster 100according to an exemplary embodiment of the present disclosure.

Referring to FIG. 2, the twin roll strip caster 100 of the exemplaryembodiment largely includes casting rolls 110, a ladle 120, a tundish130, an immersion nozzle 140, a meniscus shield 150, brush rolls 160,and edge dams 170. Reference numeral 180 denotes a strip. In a twin rollstrip casting method using such a twin roll strip caster, molten steelis directly cast as a strip having a thickness of 10 mm or less bysupplying the molten steel through an injection nozzle into a regionbetween internally-water-cooled twin rolls that are rapidly rotated inopposing directions.

An exemplary embodiment of the present disclosure provides a method ofmanufacturing a titanium (Ti)-containing austenitic stainless steelsheet using a twin roll strip caster. In the method, the composition ofmolten steel is controlled to maintain a TiN precipitation temperaturedefined by the following formula 2 at a level higher than thetemperature of the molten steel in a tundish (T/D) by at least 50° C.(ΔT).

In the exemplary embodiment of the present disclosure, a Ti-containingaustenitic stainless steel sheet manufactured by the method using a twinroll strip caster may include, by weight %, carbon (C): 0.025% to0.055%, silicon (Si): 0.25% to 0.55%, manganese (Mn): 1.5% to 1.8%,chromium (Cr): 17.1% to 17.7%, nickel (Ni): 9.25% to 9.65%, titanium(Ti): 0.2% to 0.5%, nitrogen (N): 0.025% or less, and the balance ofiron (Fe) and inevitable impurities. This composition of theTi-containing austenitic stainless steel sheet is a standard compositionwell known in the related art.

When molten steel having the above-mentioned composition is cast as astrip in a twin roll strip casting process, various casting defects maybe formed due to the influence of titanium (Ti).

Generally, materials stuck in a nozzle of a tundish and causing cloggingare TiN and TiO₂. TiN functions as nuclei or seeds, and TiO₂ gathersaround the nuclei or seeds to form a complex inclusion in the form ofclusters. For example, FIG. 3 illustrates an electron microscope imageof a TiN+TiO₂ cluster of a complex oxynitride. Therefore, it isnecessary to minimize TiN precipitation (nitride precipitation) and TiO₂formation (oxide formation).

Titanium (Ti) included in molten steel reacts with nitrogen to form aTiN inclusion. Since TiN has a high melting point of about 2000° C., TiNparticles may agglomerate together and grow in molten steel. In thiscase, the surface quality of products is markedly degraded even thoughTiN has less effect on nozzle clogging than TiO₂. Therefore, it may beimportant to prevent the formation of TiN in molten steel, especially,before the molten steel is supplied to a tundish.

Therefore, thermodynamic research has been conducted into TiN formationreaction between titanium (Ti) and nitrogen (N) expressed by thefollowing formula 1.

Ti+N=TiN  [Formula 1]

Formula 1 is disclosed in academic publications. After analyzing manyacademic materials, the following formula 2, considered to accuratelyrepresent the formation of TiN in actual Ti-containing austeniticstainless steel, is used in the present disclosure.

$\begin{matrix}{{{{Log}\left( {N\mspace{14mu} \%} \right)} = {{{- 19},{755/\left( {T + 273} \right)}} + 7.78 + {0.07\left\lbrack {\% \mspace{14mu} {Ti}} \right\rbrack} - {\log \left\lbrack {\% \mspace{14mu} {Ti}} \right\rbrack} + {0.045\left\lbrack {\% \mspace{14mu} {Cr}} \right\rbrack}}}{{T\left( {{^\circ}\mspace{14mu} {C.}} \right)} = {\frac{{- 19},755}{\begin{matrix}{{\log \left( {N\mspace{14mu} \%} \right)} - 7.78 - {0.07\left( {\% \mspace{14mu} {Ti}} \right)} +} \\{{\log \left( {\% \mspace{14mu} {Ti}} \right)} - {0.045\left( {\% \mspace{14mu} {Cr}} \right)}}\end{matrix}} - 273}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Formula 2 above expresses a TiN precipitation temperature. According toFormula 2, the TiN precipitation temperature is determined by thecontents of titanium (Ti), nitrogen (N), and chromium (Cr), and if thecontents of titanium (Ti) and nitrogen (N) reduce or the content ofchromium (Cr) increases, the TiN precipitation temperature is reduced.That is, in a caster in which solidification proceeds, the temperatureof molten steel in a tundish (T/D) has to be maintained to be higherthan the TiN precipitation temperature so as to minimize TiNprecipitation before the molten steel solidifies. Therefore, to minimizethe formation of oxynitrides (TiN and TiO₂), it may be necessary tominimize the addition of titanium (Ti) on the condition that the contentof nitrogen (N) in a refining furnace is minimized by removing nitrogen(N) and preventing the introduction of nitrogen (N).

Therefore, according to the exemplary embodiment of the presentdisclosure, the composition of molten steel is controlled to maintainthe TiN precipitation temperature defined by formula 2 at a level higherthan the temperature of the molten steel in a tundish (T/D) by at least50° C. (ΔT).

In a general continuous casting process, it is difficult to performhigh-temperature casting due to a break-out phenomenon (in whichnon-solidified molten steel breaks out of the inside of a slab due torupture of the slab). In a strip casting process, however,high-temperature casting (1550° C. or higher) is possible. Thus, adifference (ΔT) between a TiN precipitation temperature and ahigh-temperature casting temperature may be maintained to be 50° C. orgreater, and TiN precipitation may be suppressed. As a result, theformation of a complex oxynitride having TiN+TiO₂ clusters formed fromTiN seeds may be prevented, and thus nozzle clogging and surface defectsmay be minimized.

In addition, according to the present disclosure, the corrosionresistance of steel may be guaranteed by maintaining a Ti stabilizationratio (a Ti/C ratio) at a level of 8 or greater. In this case, if theaddition of titanium (Ti) is reduced, the amount of carbon (C) also hasto be reduced so as to satisfy the Ti stabilization ratio. For this, alarge amount of oxygen may be supplied to a refining furnace to decreasethe amount of carbon. As a result, a larger amount of oxygen may bedissolved in molten steel, and thus the formation of Ti oxides may beunexpectedly increased. In addition, since the period of time necessaryfor a refining process is increased, the temperature of molten steel mayincrease, and thus a considerable amount of coolant may be necessary forcooling the molten steel after the molten steel is discharged from therefining furnace. In this case, the molten steel may make contact withambient air while being cooled by the coolant and thus may bere-oxidized to cause the formation of large amounts of inclusions.

Although a larger amount of oxygen is dissolved in molten steel and theformation of TiO₂ oxide is increased because of a larger amount ofoxygen supplied to the refining furnace to remove carbon, sincesolidification occurs rapidly owing to characteristics of the stripcasting process, the size of a TiO₂ inclusion may be small or fine, andthus the influence of TiO₂ may be small. Large clusters of TiO₂ formedfrom TiN seeds are the representative form of oxide that easily sticksto a nozzle and causes surface defects.

Instead, the content of carbon (C) may be reduced to 0.3% or lowerbecause a large amount of carbon (C) is removed in the refining furnace,and thus the addition of titanium (Ti) may be reduced to satisfy the Tistabilization ratio. Therefore, the precipitation of TiN and theformation of TiO₂ may be fundamentally reduced.

Hereinafter, the exemplary embodiments of the present disclosure will bedescribed more specifically through examples.

Examples

TABLE 1 Comparative Samples (general continuous Inventive Samplescasting) (strip casting) 1 2 3 1 2 3 4 5 Molten steel C 0.025 0.03 0.0270.035 0.031 0.03 0.029 0.028 composition N 0.0147 0.015 0.013 0.0110.015 0.012 0.01 0.0091 (wt %) Ti 0.157 0.24 0.22 0.20 0.24 0.26 0.250.26 Ti/C 6.28 8.0 8.14 5.71 7.74 8.66 8.62 8.21 TiN precipitation 14871516 1501 1485 1519 1509 1493 1490 temperature (° C.) T/D molten steel1502 1510 1511 1545 1571 1559 1558 1562 temperature (° C.) CastingNozzle 20 10 5 1 2 2 1 1 results clogging (mm) *Grinding 10 8 8 — — — —— *Number of times of surface grinding (sum of counts for upper andlower surfaces)

Ti-containing austenitic stainless steel strips were manufactured usingmolten steel having compositions as shown in Table 1 above. In Table 1,Comparative Samples 1 to 3 are strips manufactured through a generalcontinuous casting process, and Inventive Samples 1 to 5 are stripsmanufactured using a twin roll strip caster. The TiN precipitationtemperature values shown in Table 1 are values calculated using theabove-described formula 2.

As shown in Table 1, in a general continuous casting process, if thetemperature of molten steel is increased due to decarbonization reactionheat as the period of time of refining increases, since high-temperaturecasting is impossible due to a break-out phenomenon (strip rupture), aconsiderable amount of coolant is supplied to decrease the temperatureof the molten steel after the molten steel is discharged from a furnace.In this case, the molten steel may easily make contact with ambient airwhile being cooled by the coolant, and thus may be re-oxidized to causethe formation of large amounts of inclusions. In addition, since castingis performed at a temperature around the TiN precipitation temperature(1490° C. to 1515° C.) due to the decreased temperature of the moltensteel, a large amount of TiN may precipitate. Therefore, the degree ofnozzle clogging in Comparative Samples 1 to 3 was high at 5 mm orgreater.

In addition, since the strips produced by the general continuous castingprocess had surface defects due to complex inclusions formed ofTiN+TiO₂, grinding was performed four or more times on each of the upperand lower surfaces of the strips. FIG. 4 illustrates an image of asurface of a cast material and an image of an inside of a tundish nozzlein a comparative example.

On the contrary, high-temperature casting (1550° C. or higher) waspossible by a twin roll strip casting method, and thus the differencebetween the TiN precipitation temperature and the temperature of castingcould be maintained to be 50° C. or greater (ΔT≧50° C.) as in InventiveSamples 1 to 5. Therefore, in Inventive Samples 1 to 5, TiNprecipitation could be suppressed to prevent the formation of a complexoxynitride having TiN+TiO₂ clusters formed from TiN seeds, and thusnozzle clogging and surface defects could be minimized.

That is, it can be understood that the method of the present disclosuresuppresses TiN precipitation to maintain the degree of tundish nozzleclogging at a level of 1 mm or lower and thus allows a casting processto be normally completed. In addition, the degree of surface quality ofTi-containing austenitic steel strips manufactured by the strip castingmethod of the present disclosure was high such that surface grinding wasnot performed.

Referring to Table 1, Inventive Samples 3 to 5 having a Ti/C ratio of 8or greater had a higher degree of corrosion resistance than InventiveSamples 1 and 2 having a relatively low Ti/C ratio.

It is necessary to confirm the purpose of Ti/C control based on thebackground of development of Ti-containing stainless steel.Ti-containing stainless steel was developed by adding titanium (Ti) toSTS304 steel as a carbon stabilizing element so as to decrease grainboundary sensitivity for use in a grain boundary sensitivity range (450°C. to 850° C.) That is, in general stainless steel, chromium (Cr) andcarbon (C) combine into Cr₂₃C₆ carbide, and a Cr-depleted zone is formedaround the Cr₂₃C₆ carbide. Therefore, the Cr-depleted zone is easilycorroded due to a relatively low chromium (Cr) content therein.Furthermore, in high-temperature applications (500° C. to 800° C.) suchas boiler heat exchangers or high-temperature pipes, corrosion occursmore rapidly due to high reactivity. Therefore, titanium (Ti) isgenerally added to stainless steel for high-temperature applications soas to cause reaction between carbon (C) and titanium (Ti) rather thanreaction between carbon (C) and chromium (Cr) and thus to fix chromium(Cr) which is an element for improving corrosion resistance. In general,a Ti/C ratio of 5 to 6 guarantees stainless steel having stablecorrosion resistance, and a higher Ti/C ratio guarantees stainless steelhaving more stable corrosion resistance.

However, in the general continuous casting method of the related art, ifthe Ti/C ratio is increased, Ti oxynitrides are excessively formedbecause a large amount of titanium (Ti) is added, and thus surfacedefects may be excessively formed. Thus it is difficult to increase theTi/C ratio.

However, in the strip casting process, although the addition of titanium(Ti) is increased to obtain a Ti/C ratio of 8 or greater, the formationof Ti oxides and TiN precipitation may be controlled and minimized toprevent surface defects. That is, the Ti/C ratio may be effectivelymaintained to be 8 or greater.

As set forth above, according to the exemplary embodiments of thepresent disclosure, nozzle clogging occurring in a general continuouscasting process may be effectively prevented, and the formation of TiNmay be suppressed. Therefore, casting stability may be guaranteed, and aTi-containing austenitic stainless steel sheet having a high degree ofsurface quality may be effectively manufactured using a twin roll stripcaster.

While exemplary embodiments have been shown and described above, it willbe apparent to those skilled in the art that modifications andvariations could be made without departing from the scope of the presentinvention as defined by the appended claims.

What is claimed is:
 1. A method of manufacturing a titanium(Ti)-containing austenitic stainless steel sheet having a high degree ofsurface quality by using a twin roll strip casting process, the methodcomprising controlling a composition of molten steel such that a TiNprecipitation temperature is higher than a temperature of the moltensteel in a tundish (T/D) by at least 50° C. (ΔT≧50° C.), the TiNprecipitation temperature being defined by the following formula 2:$\begin{matrix}{{{{Log}\left( {N\mspace{14mu} \%} \right)} = {{{- 19},{755/\left( {T + 273} \right)}} + 7.78 + {0.07\left\lbrack {\% \mspace{14mu} {Ti}} \right\rbrack} - {\log \left\lbrack {\% \mspace{14mu} {Ti}} \right\rbrack} + {0.045\left\lbrack {\% \mspace{14mu} {Cr}} \right\rbrack}}}{{T\left( {{^\circ}\mspace{14mu} {C.}} \right)} = {\frac{{- 19},755}{\begin{matrix}{{\log \left( {N\mspace{14mu} \%} \right)} - 7.78 - {0.07\left( {\% \mspace{14mu} {Ti}} \right)} +} \\{{\log \left( {\% \mspace{14mu} {Ti}} \right)} - {0.045\left( {\% \mspace{14mu} {Cr}} \right)}}\end{matrix}} - 273}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$
 2. The method of claim 1, wherein the molten steelcomprises, by weight %, carbon (C): 0.025% to 0.055%, silicon (Si):0.25% to 0.55%, manganese (Mn): 1.5% to 1.8%, chromium (Cr): 17.1% to17.7%, nickel (Ni): 9.25% to 9.65%, titanium (Ti): 0.2% to 0.5%,nitrogen (N): 0.025% or less, and the balance of iron (Fe) andinevitable impurities.
 3. The method of claim 1, wherein the moltensteel has a Ti/C ratio of 8 or greater.